Early intensive behavioral intervention is the only treatment for autism spectrum disorders for which a substantial number of studies have demonstrated efficacy. However, not all children respond to this treatment, and recent evidence suggests that success and failure are determined more by variations among patients than by variations among intensive intervention programs. To choose the best treatment for each child, it is important to determine which characteristics of children with ASDs predict their response to behavioral intervention. Further, since it is likely that responders and non-responders differ in etiology, successful categorization on this dimension could provide a way of stratifying children with ASDs into genetically meaningful subsets. Thus, successful prediction would serve both an important public health goal and an important basic science goal. Because many children in early intervention programs in our region receive very similar programs of ABA therapy at public expense, we have a population in which this question can be addressed at a reasonable cost. Over the first 2 years of intervention, response to treatment will be measured by change in four domains: behaviors related to autism, language, cognition, and adaptive behavior. Measures of four possible predictor variables will be tested for their correlation with change. These are: aloof social style, obsessive self stimulation of the senses, rudimentary receptive language, and imitation. The hypothesis is that the first two will predict low levels of response, and the last two will predict high levels of response to treatment. Each participant will be genotyped for alleles of six candidate genes (HOXA1, HOXBI, RELN, WNT2, HOXD1, GBX2), to determine whether either scores on the predictor variables or change scores are correlated with suspected genetic susceptibility factors for ASDs.